I would think nothing would happen. Just let it thaw out. The head pressure in the keg would go up since all the CO2 would come out but the kegs are rated to around 100psi so I would think they could take it. Plus they have the pressure relief valve.

As for the beer, it would kill all the remaining yeast but at that point who cares. It should not harm the beer in any other way.

Depending on how full the keg is, you also have to account for the expansion of water as it freezes, which could put quite a bit of force on the walls of the keg. I could imagine a full keg getting quite deformed if it froze solid.

I don't know why but no one ever mentions a carbonation stone. You can carb a beer in less than 24 hours with one.

Probably because the only way a stone could speed up the process would be to bubble through a non-pressurized vessel for the duration, wasting a sh!tload of gas in the offing. Otherwise, if the vessel is sealed, in short order it would reach equilibrium with the regulator and no more gas would enter it, and you'd be barely better off than without a stone.

You can carbonate a keg in an hour without a stone if you put it on gas and rock the keg for the duration. There's nothing magical about using a stone in that case...

That manifold will allow you to direct gas to 3 separate kegs. The pressure will set by the regulator which is most likely going to be placed between that manifold and the CO2 tank, soo.... No, your pressure would be the same on all 3 lines. I also HIGHLY doubt that manifold is good for the full cylinder pressure without having a regulator between the manifold and cylinder.

All, for what its worth, I maintain my beer in my keezer set to 28 degrees F. It modulates between 28 and 32 degrees F, and in the past I have noticed slight icing after i thought I had floated the keg and opened it up to find out I have a nice layer of ice remaining in the bottom, but never noticed any issues with freezing solid. Not everyone prefers to keep their beer that cold, but just my $0.02 that a temperature like that has not ever caused any adverse affects. On a side note, the icing issue only happened on one keg and i believe i had my keezer colder than it currently sits. My last keg (floated 2 days ago) had zero icing.

Depending on how full the keg is, you also have to account for the expansion of water as it freezes, which could put quite a bit of force on the walls of the keg. I could imagine a full keg getting quite deformed if it froze solid.

I've frozen my kegs twice now due to a faulty Ranco controller. No damage to the kegs and the beer was the same once it thawed. I guess I had enough head space to not destroy my kegs with bulging ice hopsicles.